Most questions on Stack Exchange have been assigned one or several tags by Stack Exchange users. Are these assignments of tags to Stack Exchange questions licenseable?
Under US copyright law, I do not think this is copyrightable. See the US Copyright Office's Compendium of the U.S. Copyright Office Practices, Third Edition, Chapter 300, Section 313.4(B). The Supreme Court held in Feist v. Rural Telephone (499 US 340, 1991) that "copyright protects only those constituent elements of a work that possess more than a de minimis quantum of creativity".
The Compendium gives some examples of works that do not meet this standard, some of which seem analogous to a set of tags on an existing post:
A Venn diagram consisting of three overlapping circles containing the names of various personality disorders and a few words and short phrases that describe the symptoms of each condition.
A synopsis [of a work of the performing arts] consisting of a single sentence.
A public domain photograph of Winston Churchill combined with the word “Commitment” and the quotation “Never, never, never give up.”
Since the tagging is not subject to copyright, the concept of licensing makes no sense: anyone is free to copy such a "work" regardless of whether or not the "author" has granted them a license, and doing so would not bind them to any conditions imposed in any such license.